The evaluation electronics of a Coriolis mass flow sensor manufactured by the applicant are described in the journal "Automatisierungstechnische Praxis--atp", 1988, pages 224 to 230. This mass flow sensor has a first measuring tube vibrating at a vibration frequency and a second measuring tube vibrating at the same vibriation frequency, the measuring tubes being traversed by a fluid to be measured, and a first optoelectronic transducer and a second optoelectronic transducer being positioned along the measuring tubes at a distance from each other in the direction of flow, the evaluation electronics comprising:
a light-emitting diode and a photodiode per optoelectronic transducer; PA1 a difference stage for the output currents of the two transducers; PA1 an amplifier succeeding the difference stage; PA1 an adjustment stage PA1 a sequencer which keeps the switch ON during an ON period except during a measurement period; PA1 an amplitude-measuring stage having its input connected to the output of the difference stage; PA1 a memory stage having its input connected to the output of the amplitude-measuring stage at least during the measurement period; and PA1 a processor stage whose output is proportional to mass flow rate. PA1 a first, fixed-gain amplifier for the signal from the first transducer; PA1 a second, variable-gain amplifier for the signal from the second transducer; PA1 a difference amplifier for the output signals from the two amplifiers; PA1 an active filter succeeding the difference amplifier and serving to eliminate measurement errors which are caused by variations in the density of fluid, by the vibration frequency itself, and by the excitation of the measuring tubes; PA1 a summing amplifier for the output signals from the two amplifiers; PA1 a first lock-in amplifier, locked onto the vibration frequency, at the output of the active filter; PA1 a second lock-in amplifier, locked onto the vibration frequency, at the output of the summing amplifier; PA1 a first voltage-to-frequency converter at the output of the first lock-in amplifier and a second voltage-to-frequency converter at the output of the second lock-in amplifier; PA1 a first counter at the output of the first voltage-to-frequency converter and a second counter at the output of the second voltage-to-frequency converter; and PA1 a divide stage PA1 a first preamplifier for the signal from the first transducer; PA1 a second preamplifier for the signal from the second transducer; PA1 a first, fixed-gain amplifier for the output signal from the first preamplifier; PA1 a second, variable-gain amplifier for the output signal from the second preamplifier; PA1 a difference stage for the output signals from the two amplifiers; PA1 a changeover switch PA1 a variable-cutoff-frequency low-pass filter PA1 a summing/integrating stage for the output signals from the two preamplifiers; PA1 a phase-locked loop PA1 a phase-measuring and phase-adding stage PA1 a first synchronous rectifier clocked by the second vibration-frequency signal and fed with the output signal from the low-pass filter; PA1 a switch having its input connected to the output of the first synchronous rectifier; PA1 an adjustment stage for adjusting the gain of the second amplifier and having its input connected via a first integrator to the output of the switch; PA1 a sequencer which keeps the switch ON during an ON period and the output of the changeover switch connected to the first input of the changeover switch except during a measurement period which is short compared with the ON period and during which the sequencer activates the phase measurement of the phase-measuring and phase-adding stage; PA1 a second synchronous rectifier which is clocked by the second vibration-frequency signal during the measurement period and by the third vibration-frequency signal during the ON period, and which is supplied with the output signal from the low-pass filter; PA1 a second integrator having its input connected to the output of the second synchronous rectifier; PA1 a memory stage having its input connected to the output of the second integrator during the measurement period at the most; and PA1 a divide stage
whose input is connected to the output of the amplifier, PA2 whose output is connected via a switch to the photodiode of the second transducer, and PA2 by means of which the amplitude of the output current of the second transducer is adjusted to be equal to the amplitude of the output current of the first transducer; PA2 whose dividend input is connected to the output of the first counter, PA2 whose divisor input is connected to the output of the second counter, and PA2 whose output signal is proportional to mass flow rate. PA2 whose first input is connected to the output of the difference stage, and PA2 whose second input is connected to the output of the first amplifier; PA2 whose transmission begins to decrease at the vibration frequency of the measuring tubes, and PA2 whose input is connected to the output of the first changeover switch; PA2 whose input is connected to the output of the summing/integrating stage, and PA2 whose output signal is a first vibration-frequency signal; PA2 whose first input is supplied with the first vibration-frequency signal, PA2 whose second input is supplied with the output signal from the low-pass filter, PA2 whose first output provides a second vibration-frequency signal shifted in phase by an amount equal to the phase shift of the low-pass filter, and PA2 whose second output provides a third vibration-frequency signal shifted in phase by an amount equal to said phase shift plus 90.degree.; PA2 wherein during the ON period PA2 whose output signal is proportional to mass flow rate.
The journal "Measurements & Control", September 1988, pages 195 to 197, describes the evaluation electronics of a Coriolis mass flow sensor having a first measuring tube vibrating at a vibration frequency and a second measuring tube vibrating at the same vibration frequency, the measuring tubes being traversed by a fluid to be measured, and a first electrodynamic transducer and second electrodynamic transducer being positioned along at least one of the measuring tubes at a distance from each other in the direction of flow, the evaluation electronics comprising:
It is an object of the invention to provide improved evaluation electronics for Coriolis mass flow sensors comprising electrodynamic transducers, so that at least an accuracy of 0.005% of the measuring-range limit value (e.g., at a fluid velocity of 10 m/s) and an accuracy of 0.15% of the measured value are attainable. These values also mean that a fluid velocity of 0.5 mm/s must still be measurable. The phase difference to be measured between the signals from the two electrodynamic transducers ranges between approximately 4.multidot.10.sup.-5 .degree. and approximately 0.5.degree. at vibration-frequency values between approximately 500 Hz and approximately 1 kHz. The phase measurement must therefore be made with a resolution of approximately 2.multidot.10.sup.-10 seconds.